It is commonly understood in situations where a rotatable arm of a robotic manipulator, or similar rotated lever mechanism, is rotated in a particular direction about an axis and held in place at such rotated position, that a torque load must be imposed at the axis of rotation in the direction opposite to the rotational movement of such arm or mechanism in order to hold the arm in such position. In particular, when a cantilevered robotic arm is rotated about an axis, a certain amount of rotational torque is required to maintain such robotic arm in rotated position (e.g., at a particular angle relative to the vertical or horizontal). If the robotic arm is relatively heavy, extends a substantial distance from the axis of rotation, and/or carries a payload at its distal end, the torque force required to maintain such arm in rotated position can be quite substantial. In this regard, circumstances may dictate the use of a much larger rotating motor or torque producing device simply to compensate for the substantial loads bearing on the rotation axis in use. Numerous attempts in the industry to use hydraulic or pneumatic devices to reduce the loads bearing on the driving motors of various rotatable robotic arms generally required undesirably large and complicated devices. As robotic manipulators are often used in manufacturing applications where working space is at a premium, cumbersome devices impose critical drawbacks. Moreover, added complexity not only adds to the cost of the robotic manipulator, but further adds to the cost of the maintenance thereof.
A robotic arm having a bearing support designed to overcome the size and complexity problems described above is shown in U.S. Pat. No. 4,546,233, which issued to H. Yasuoka on Oct. 8, 1985. In particular, the Yasuoka patent discloses a robotic arm having a bearing support comprising a tension spring provided between a tiltable upper arm and a stationary robot base. The upper arm is supported tiltably on top of a rotary table which is mounted on the stationary table. A tension spring has one end fastened to the lower end of the tiltable upper arm, and its other end supported rotatably on the stationary table. Such tension spring is, therefore, tiltable and rotatable to correspond with the tilting and rotation of the upper arm, such that the spring remains in proper position to provide tension which urges the lower end of the upper arm in a direction opposite to that in which the arm has been tilted. In this manner, the Yasuoka bearing support reduces the load imposed on the motor for tilting the upper arm, without requiring large and complicated hydraulic or pneumatic load reducing devices. As can be seen from a review of the Yasuoka reference, however, the tension spring device itself requires a relatively substantial area within the center of the robotic device. It is often required (or at least desired) to route power lines and/or product supply lines (as appropriate) through the center of the robotic device for safety reasons and to minimize the space requirements of a robot. The rotating tension spring device shown in Yasuoka would make such use of the central portions of the robot difficult, if not impossible. Additionally, tension springs have a relatively short useful life and can fail catastrophically.
Consequently, despite all the prior work done in the industry in an effort to reduce the load bearing on a motor or other torque producing device in rotatable arm assemblies and the like, there remain problems of providing a dependable load reducing device which minimizes the cost, space requirements and interference with other parts and operations of the robotic manipulator. With prior art devices, it was necessary to incorporate cumbersome hydraulic or pneumatic devices, or utilize tension springs or counterbalancing weights which also require substantial space and have a relatively limited useful life.